The invention herein described was made in the course of or under a contract, or subcontract thereunder with the Department of the Army.
When radiation of the proper energy falls upon a photoconductive semiconductor, the conductivity of the semiconductor increases. Energy supplied to the semiconductor causes covalent bonds to be broken, and electronhole pairs in excess of those generated thermally are created. These increased current carriers decrease the resistance of the material. This "photoconductive effect" in semiconductor materials is used in photoconductive detectors.
A photoconductive detector is a bar of semiconductor material having electrical contacts at the ends. In its simplest form, the photoconductive detector is connected in series with a direct current voltage source and a load resistor. The change in resistivity of the photoconductive detector in response to incident radiation is sensed in one of two ways. If the resistance of the load resistor is much greater than the resistance of the detector, the device is operated in the "constant current mode," since the current through the detector is essentially constant. In this mode, the change in resistivity of the photoconductive detector is usually sensed by measuring the change in voltage across the photoconductive detector.
If, on the other hand, the resistance of the load resistor is much less than the resistance of the detector, the photoconductive detector is operating in the "constant voltage mode" since the voltage across the photoconductive detector is essentially constant. The change in resistivity of the photoconductive detector is usually sensed by measuring the voltage change across the load resistor.
Of the two detector modes, the constant current mode finds wider use in photoconductive detectors made from semiconductor materials having low resistivity. For this reason, further discussion in this specification will deal with the constant current mode rather than the constant voltage mode.
Photoconductive detectors have found many applications. One particularly important area is in the detection of infrared radiation. Infrared sensing photoconductive detectors are widely used for various heat and object sensing applications.
One widely used intrinsic infrared sensitive photodetector material is mercury cadmium telluride, which consists of a mixture of cadmium telluride and mercury telluride. Cadmium telluride is a wide gap semiconductor (E.sub.g =1.6eV), and mercury telluride is a semimetal having a "negative energy gap" of about -0.3eV. The energy gap of the alloy varies linearly with x, the mole fraction of cadmium telluride in the alloy, Hg.sub.1-x Cd.sub.x Te. By properly selecting x, it is possible to obtain mercury cadmium telluride detector materials having a peak response at any of a wide range of infrared wavelengths. Of particular importance are those wavelengths in the 8 to 14 micron range.
When signal producing radiation impinges upon a photoconductive body, a signal is produced from the resultant flow of electrons. The photoconductive gain is equivalent to the number of times an electron passes through the circuit before the electron recombines with one of the holes created. This length of time while the electron is passing through the circuit is known as a recombination time, generally defined as how long the electron will travel before it recombines with a hole. This recombination time is directly related to the temperature of the semiconductor. In many semiconductors, and in particular, with mercury cadmium telluride, it is necessary to operate the semiconductor at a cryogenic temperature of, for example 3.degree.K to 80.degree.K in order to have a recombination time sufficient to achieve a usable photoconductive gain.
Materials such as mercury cadmium telluride which have extremely long photoconductive response times, on the order of 100 to 300 milliseconds, have not been found to be useful in certain applications because of this long recovery time. Many times, the photoconductive material is in an atmosphere where nuclear or gamma ray sources are present, either naturally or created artificially. Thus, nuclear interactions will swamp the detector and no useful output can be obtained. Since the detector must receive radiation from the source sought to be detected, it is impossible to completely shield the detector from unwanted nuclear radiation. In other instances, various objects are sought to be detected such as, for example, two closely spaced stars or other celestial bodies. If these two objects to be detected are too close together, the detector will still be reporting the signal of the first body when it begins to receive the radiation from the second body. Many times, the two signals either overload the detector or, in the case when the second body is significantly smaller than the first, the second signal is lost in the response from the first body's signal.
Accordingly, it would be of great advantage to the art if a semiconductor device could be created which would be sensitive, that is have long recombination times and therefore a high photoconductive gain, without the adverse effects of a long recombination time.
Another advantage would be if a detector could be provided which would be insensitive to background (nuclear) radiation of the type which produces more signals than can be effectively dissipated.